Re: [PATCH] blk-iocost: use irq-safe locking in cgroup handlers

[Jens Axboe <axboe@kernel.dk>]

On 6/1/26 3:50 PM, Bart Van Assche wrote:
> On 5/31/26 11:13 PM, Yu Kuai wrote:
>> @@ -3378,14 +3378,14 @@ static u64 ioc_cost_model_prfill(struct seq_file *sf,
>>       if (!dname)
>>           return 0;
>>   -    spin_lock(&ioc->lock);
>> +    spin_lock_irq(&ioc->lock);
>>       seq_printf(sf, "%s ctrl=%s model=linear "
>>              "rbps=%llu rseqiops=%llu rrandiops=%llu "
>>              "wbps=%llu wseqiops=%llu wrandiops=%llu\n",
>>              dname, ioc->user_cost_model ? "user" : "auto",
>>              u[I_LCOEF_RBPS], u[I_LCOEF_RSEQIOPS], u[I_LCOEF_RRANDIOPS],
>>              u[I_LCOEF_WBPS], u[I_LCOEF_WSEQIOPS], u[I_LCOEF_WRANDIOPS]);
>> -    spin_unlock(&ioc->lock);
>> +    spin_unlock_irq(&ioc->lock);
>>       return 0;
>>   }
> 
> This change is wrong. ioc_cost_model_prfill() only has one caller,
> namely blkcg_print_blkgs(). blkcg_print_blkgs() calls the above function
> with interrupts disabled. The spin_unlock_irq(&ioc->lock) at the end of
> the above function enables interrupts while q->queue_lock is held. If an
> interrupt happens on the same CPU core before q->queue_lock is unlocked,
> and that interrupt tries to lock q->queue_lock, a deadlock will occur.

Agree, it's broken. Which makes me suspect of the traces shown. Yu,
can you please shed some light on this?

I've dropped it, thanks Bart.

-- 
Jens Axboe